The possibility of using algae for the production of fuel and chemicals has attracted the interest of researchers, government and business for many years. Efforts to commercialize the production of fuel from algae have brought to light problems that must be solved to make this approach practical. The present invention is a novel approach to avoid or mitigate certain problems.
Under conventional approaches to algal biofuels, algal biomass is accumulated in open ponds or photobioreactors and harvested for conversion to fuel. The composition of the biomass may be altered to some extent through manipulation of the organism genetics or of the environment in which the organism is cultured, but generally there is a trade-off between optimizing composition and maximizing accumulation of biomass.
Under an alternative approach, the genetics and environment of a photosynthetic organism are manipulated to force the flux of carbon through photosynthesis into a desired product instead of toward accumulation of biomass. In the present invention, photosynthetic organisms are cultured as a biofilm inside a photobioreactor and maintained in stationary phase, and the environmental conditions in the photobioreactor are manipulated to induce the organisms to make a biofuel product, such as ethanol.
Obligate photosynthetic organisms require CO2 as a feedstock to make a product or to accumulate biomass. One problem is that, if a photosynthetic organism is cultured in a body of water in a photobioreactor or pond that is exposed to CO2 contained in air, then passive diffusion of carbon from air into water across the gas/liquid interface generally is not as efficient in sustaining maximal rates of photosynthesis. Accordingly, supplemental CO2 usually must be added to cultures of photosynthetic organisms that are highly productive. The use of supplemental CO2 significantly increases capital costs and operating costs, thereby reducing the profitability and rate of return of CO2-supplemented systems.
Limited diffusion may be ameliorated by increasing the exposure of the culture of photosynthetic organisms to air, beyond the degree of exposure found with a culture contained in a body of water. Diffusion of CO2 into a culture increases as the surface area of the culture that is in contact with air increases. In addition, many organisms express carbonic anhydrase at the cell surface, which catalyzes the conversion of carbon dioxide and water to bicarbonate and protons and thereby increases the rate of diffusion of CO2 from the gas phase into the liquid phase:

As a result, an air-exposed culture of an appropriate organism can achieve high productivity without CO2 supplementation.
In many circumstances, gas phase delivery of CO2 is preferred over other delivery modes. Because the concentration of CO, in air is typically less than 0.1% by volume, sustained elevated productivity by a culture requires considerable air throughput. The present invention provides a photobioreactor with a short airflow path and sufficiently low resistance to airflow that the necessary throughput of air can be achieved without high capital or operating costs. In some circumstances, the most economical delivery of CO2 may be from a concentrated source instead of from the air. In some circumstances the most economical delivery of CO2 may be as a solution of bicarbonate. The present invention provides a photobioreactor compatible with all of these modes of CO2 delivery.
Another problem is that highly productive photosynthetic cultures tend to accumulate oxygen, which is a product of oxygenic photosynthesis. High oxygen concentrations in the culture can reduce productivity both by competing for photosynthetically produced electrons and through the effects of oxygen toxicity.
In a conventional photobioreactor containing a liquid suspension culture, oxygen may be removed from the culture by vigorous gas sparging, but high energy costs may be involved. This consideration militates in favor of gas phase exchange a preferred method of removing excess oxygen from the culture. The present invention facilitates gas phase exchange by providing a very short diffusion path for oxygen removal from the culture to the air stream.
Another problem is that an organism that channels photosynthetic energy primarily into making a fuel or chemical product, instead of accumulating biomass, severely disadvantaged compared to a competing organism that does not make the fuel product, and instead channels photosynthetic energy toward growth. This disparity reduces the stability of a culture of organisms that make a fuel product, since the culture may be invaded and outcompeted by other species that do not make the fuel product. Also, the organism will undergo mutations that reduce the tendency to make the fuel product, thereby conferring a selective advantage over the productive, non-mutant type. As a result, the non-productive mutants will take over the culture, reducing or eliminating the productive organisms. This problem can be mitigated if production of the fuel or chemical product is beneficial to the organism.
If the organism makes the fuel product through fermentation, then production of the fuel product is necessary for the metabolism of the organism under anaerobic conditions and consequently the culture is more stable against mutation of the organism or invasion by non-fermenting species. The present invention facilitates fermentation to make a fuel product.
Product stability can be problematic if the fuel or chemical product is present in the culture and oxygen is present in the culture. The growth of aerobic heterotrophic bacteria that consume the product and that are present in the culture as contaminants is enabled by the availability of both oxygen and the product.
To address this problem, conventional photobioreactor or pond cultures must either incorporate unbreachable sterility barriers or must use antibiotics or other means so they are tolerant of some degree of contamination. The present invention minimizes the effect of contamination by heterotrophs on product stability and net productivity by substantially removing the product so that it is not present when oxygen is present in the culture.
Toxic effects of products such as ethanol on the organism of interest may also present problems. While product toxicity increases with productivity and product concentration, product toxicity can be mitigated by limiting the duration of exposure of the organism to the maximum product concentration. Further, exposure of the culture to product can be limited to the fermentation period, which may be conducted in darkness, in order to avoid toxicity responses that result from an interaction with photosynthetic processes.
Product purification costs are usually sensitive to concentration of the product that is extracted from a culture in a photobioreactor. It is desirable for fermentation to occur in a small fluid volume that yields elevated product concentration. The present invention provides a photobioreactor in which the fermentation volume is very small.
Capital costs must be kept within reasonable bounds for a fuel production system or method to be economically feasible. Materials, construction methods and supporting infrastructure must be chosen or designed with low cost in mind. A system of the present invention can have low material costs and a simplified infrastructure, and may be made using simple construction methods suitable for mass production. A system of the present invention may be light weight, minimizing mounting costs.
Because photosynthetic organisms in photobioreactors require exposure to sunlight, the culture in a photobioreactor may be exposed to high temperatures that are inimical to culture health and productivity. The present invention allows the management of culture temperature at low cost.
The considerations outlined above illustrate that the productivity of organisms that are cultured in a photobioreactor to make biofuel through metabolic processes may be restricted severely by limitations on uptake of CO2 by the culture, removal of oxygen from the culture, genetic stability of the culture, stability of the product made by the culture, toxicity effects of the product on the culture and temperature effects on the culture.
US 2009/0181434 A1 to Aikens et al. discloses transgenic bacteria engineered to accumulate carbohydrates and a photobioreactor for cultivating photosynthetic microorganisms comprising a non-gelatinous, solid cultivation support suitable for providing nutrients and moisture to photosynthetic microorganisms and a physical barrier covering at least a portion of the surface of the cultivation support.
Aikens does not provide for the possibility of anoxic fermentation in the reactor structure or mode of operation. The photobioreactor proposed by Aikens is very different in detail from the present invention, using a different medium delivery system, a different product harvest system, and a completely different mode of operation. It does have in common with the present invention the use of a photosynthetic biofilm. The advantages of the present invention are that (1) periodic immersion provides a much more reliable uniform hydration than water seeping or dripping from a header; (2) the complexity and cost of a reactor design of the present invention are much lower; and (3) a reactor design of the present invention design lends itself to easily establishing conditions suitable for fermentation.
US 2008/0160591 A1 to Wilson at al. discloses a photobioreactor system for production of photosynthetic microorganisms that includes the use of extended surface area and an external water basin. Wilson et al. is related to the present invention in that Wilson at al. teaches the use of plastic film and similar construction techniques to produce a pattern of heat sealed welds between opposite panels. This reflects the concern of Wilson at al. with reactor cost, which is a concern also addressed by the present invention. Wilson et al. provides a photobioreactor design that is suited for the cultivation of organisms suspended in water medium. Wilson at al. is not suitable for cultivation of a photosynthetic biofilm, and hence it does not provide the separation of retained biomass from a secreted or soluble product, and it is not suitable for operation with an autofermentation cycle.
US 20090258404 A1 to Mikkelsen at al. discloses production of fermentation products such as ethanol and lactic acid in biofilm reactors by microorganisms immobilized on sterilized granular sludge. Mikkelsen et al. is similar to the present invention in that Mikkelsen at al. uses a biofilm and anoxic fermentation. The apparatus and method of Mikkelsen at al. are not suitable for a photosynthetic biofilm or for an alternation of photosynthesis and autofermentation conditions essential to the present invention.
U.S. Pat. No. 5,595,893 to Pometto at al. discloses a solid support made of a synthetic polymer for immobilization of microorganism cells to form a biofilm reactor or use in fermentation, in streams for bioremediation of contaminants, and in waste treatment systems. It is possible that the support specified by Pometto et al, would be useful in a photobioreactor of the present invention. The reactor design used by Pometto et al. and the method of use are not compatible with a photosynthetic biofilm and an alternation of photosynthesis and autofermentation conditions essential to the present invention.
These references do not teach an optimized biofilm photobioreactor system of the present invention that resolves the limitations discussed above.